1. Field of the Invention
The present invention relates to a process for evaluating a thermal cycle life of an article to be subjected to a thermal cycle under actual service conditions, based on restraint rates at various points on the article.
2. Description of the Prior Art
A process for evaluating a thermal cycle life of an exhaust manifold subjected to a thermal cycle using, as parameters, an offset yield strength of the exhaust manifold at a lower limit temperature in the thermal cycle and an area of a plastic strain loop in a second thermal cycle, is disclosed in xe2x80x9cHitachi Metal Technical Reportxe2x80x9d, Vol. 8, pp. 79-84. A process for evaluating a thermal cycle life of an exhaust manifold using a largest value of a plastic strain of the exhaust manifold as an evaluating parameter is disclosed in xe2x80x9cPrevious Print Issue, No.9833115 in Scientific Lecture Meeting by Society of Automotive Engineers of Japan, Inc.xe2x80x9d.
In the above conventional process, it is necessary to provide a thermal cycle under actual service conditions to the exhaust manifold to actually measure variations in strain and stress with the passage of time. However, when the highest temperature under the actual service conditions is high, the strain and stress of the exhaust manifold cannot be measured and for this reason, the evaluating parameter cannot be determined directly from a measured value. There is also a process involving the reproduction of the evaluating parameter using a simulation provided by a computer. However, in a state in which a measured value in an exhaust manifold cannot be obtained due to a high temperature, a simulation result cannot be collected and for this reason, an evaluating parameter having a high accuracy cannot be obtained.
The present invention has been accomplished with the above circumstance in view, and it is an object of the present invention to accurately evaluate a thermal cycle life of an exhaust manifold by obtaining a restraint rate as an evaluating parameter.
To achieve the above object, there is provided a process for evaluating a thermal cycle life of an exhaust manifold, which will be described below.
The present invention is directed to a process for evaluating a thermal cycle life of an article to be subjected to a thermal cycle under actual service conditions, based on restraint rates at various points on the article, comprising the steps of providing a thermal cycle of a temperature profile under evaluating conditions to the article, so that the temperature profile under actual service conditions and the temperature profile under the evaluating conditions satisfy the following condition expression:
|[t(x)xe2x88x92t0]/[t(x=i)xe2x88x92t0]xe2x88x92[T(x)xe2x88x92T0]/[T(x=i)xe2x88x92T0]| less than E
wherein t(x) is the temperature at a site X when at a high temperature in the thermal cycle under the actual service conditions; t0 is the equalized temperature when at a low temperature in the thermal cycle under the actual service conditions; t(x=i) is the temperature at a site i when at the high temperature in the thermal cycle under the actual service conditions; T(x) is the temperature at the site X when at the high temperature in the thermal cycle under the evaluating conditions; T0 is the equalized temperature when at the low temperature in the thermal cycle under the evaluating conditions; T(x=i) is the temperature at the site i when at the high temperature in the thermal cycle under the evaluating conditions; and E is a predetermined value. The thermal cycle life of the article to be subjected to the thermal cycle under the actual service conditions is evaluated, based on the restraint rates determined in the temperature profile.
The term xe2x80x9cevaluating conditionsxe2x80x9d means a temperature at which a restraint rate can be measured by a strain gauge, e.g., 300xc2x0 C. The term xe2x80x9cactual service conditionsxe2x80x9d means a temperature at which a restrained rate cannot be measured by the strain gauge, e.g., 600xc2x0 C. to 750xc2x0 C. The term xe2x80x9cequalized temperaturexe2x80x9d when at the low temperature in the thermal cycle means ambient temperature, e.g., 25xc2x0 C.
If the absolute value of a deviation between the standardized temperature profile [T(x)xe2x88x92T0]/[T(x=i)xe2x88x92T0] of the article under the evaluation conditions and the standardized temperature profile [t(x)xe2x88x92t0]/[t(x=i)xe2x88x92t0] of the article under the actual service conditions is smaller than the predetermined value E, it is ensured that a restraint rate xcex7 under the actual service conditions and a restraint rate xcex7 under the evaluating conditions are substantially equal to each other. Therefore, a restraint rate xcex7 of the article can be determined only by providing, to the article, the thermal cycle under the evaluating conditions in which a thermally expanded amount can be measured by the strain gauge, on condition that the absolute value of the deviation is smaller than the predetermined value, without providing, to the article, the thermal cycle under the actual service conditions in which it is difficult to measure a thermally expanded amount by the strain gauge due to a high temperature. Thus, the thermal cycle life of the article can be evaluated precisely based on the restraint rate xcex7.